In television telephone or television conference, in order to transmit a moving image efficiently even in a narrow transmission route, the moving image is digitized, and this signal is further processed by digital image compression, and is transmitted, and at the receiving side, the compressed digital moving image signal is decoded, and the moving image is reproduced.
In a video disc or the like, in order to accumulate more moving images in one disc, the digital moving image signal is coded and the image is compressed.
The conventional coding method and decoding method of digital moving image signal include the ITU-T Recommendation H. 261 recommended in March 1993, and the MPEG by the ISO, and the ITU-T Recommendation is mainly described in this specification. For the sake of convenience, the coding method is first described, and then the decoding method, by referring to FIG. 4 to FIG. 6.
The coding method of H. 261 is realized, as shown in FIG. 4, by a coding apparatus comprising motion vector detecting means 401, motion compensation means 402, prediction error calculating means 403, intra-inter judging means 404, DCT 405, quantizing means 406, variable length coding means 407, inverse quantizing means 408, inverse DCT 409, reconstruction means 410, frame memory 411, and intra/inter switch 412.
Each constituent element of H. 261 coding method is described below. First, the motion vector detecting means 401 detects the motion vector between a previous reconstruction frame stored in the frame memory 411, and an entered present frame. The motion vector is detected, as shown in FIG. 5, in a unit of pixel block called macro block. The motion vector may be regarded as a spatial displacement amount of pixel block between the previous reconstruction frame and the present frame. Next, the motion compensation means 402 displaces the pixel block of the previous reconstruction frame stored in the frame memory 411 according to the motion vector, and generates a predicted image. This motion compensation is a process for reducing the prediction error in the prediction error calculation at a next step.
The prediction error calculating means 403 calculates the differential value between the macro block of the entered present frame and the predicted image in the pixel unit, and issues as a prediction error. Further, both the macro block of the present frame and the prediction error are entered into the intra/inter judging means 404. Herein, for example, by comparison between the variance of pixels in the macro block of the present frame and the variance of the prediction error, it is judged whether coding is more efficient by directly coding the macro block of the present frame (intra-coding), or coding is more efficient by coding the prediction error (inter-coding), and according to the result, either the macro block of the present frame or the prediction error is issued. At the same time, the result of judgement is issued as an intra/inter control signal.
The macro block of the present frame or the prediction error issued from the intra/inter judging means 404 is transformed into a frequency region by the DCT 405 and is further transformed into a quantizing coefficient by the quantizing means 406 to be curtailed in redundancy, and further the quantizing coefficient is transformed into a variable length code on the basis of the statistic character by the variable length coding means 407, and the quantity of information is curtailed. In the variable length coding means 407, further, the intra/inter control signal and motion vector are entered, and coded into variable length codes. The variable length coding means 407 multiplexes all these variable length codes, and issues as a bit stream.
On the other hand, the quantizing coefficient which is the output of the quantizing means 406 is inversely quantized by the inverse quantizing means 408, and is inversely transformed from the frequency region by the inverse DCT 409. When the macro block being presently coded is intra-coding, the intra/inter switch 412 is turned off, and the output from the inverse DCT 409 is directly stored in the frame memory 411. To the contrary, when the macro block being presently coded is inter-coding, the intra/inter switch 412 is turned on, and the output of the inverse DCT 409 and the output of the motion compensation means 402 are added in the reconstruction means 410, and stored in the frame memory 411. Thus, the present reconstruction frame is stored in the frame memory 411, and it is used in prediction of next frame.
Next, the decoding method of H. 261 is realized by an apparatus, as shown in FIG. 6, comprising variable length code decoding means 601, inverse quantizing means 602, inverse DCT 603, frame memory 604, motion compensation means 605, intra/inter switch 606, and reconstruction means 607.
Each constituent element of the H. 261 decoding method is described below. First, the variable length code decoding means 601 separates and decodes the variable length code from the entered bit stream, and issues the quantizing coefficient, intra/inter control signal, and motion vector in the macro block unit. The quantizing coefficient is inversely quantized by the inverse quantizing means 602, and is inversely transformed from the frequency region by the inverse DCT 603. The motion compensation means 605 displaces the pixel block of the previous reconstruction frame stored in the frame memory 604, on the basis of the motion vector, and generates a predicted image.
When the intra/inter control signal instructs intra-coding, the intra/inter switch 606 is turned off, and the output from the inverse DCT 603 is directly stored in the frame memory 604. On the other hand, when the intra/inter control signal instructs inter-coding, the intra/inter switch 606 is turned on, and the output of the inverse DCT 603 and the output of the motion compensation means 605 are added in the reconstruction means 607, and stored in the frame memory 604. Thus, the present reconstruction frame is stored in the frame memory 604, and it is used in prediction of next frame and is also issued as a decoded image.
In the coding method and decoding method of H. 261, the inverse quantizing means, inverse DCT, and motion compensation means process the same, and therefore the reconstruction frames stored in the frame memory of the coding method and decoding method always coincide with each other.
Generally, since the bit stream issued by the coding method of moving image signal is composed of variable length codes, in case an error occurs in transmission or accumulation, if it is an error of one bit, the decoding error occurs in a wide range, and the picture quality of the reconstruction frame deteriorates. In the conventional decoding method of moving image signal, the reconstruction frame lowed in picture quality due to decoding error is used in prediction of next frame, this picture quality deterioration can propagate to the succeeding frames later in time.
As one of the prior arts presenting a method of solving this problem, “Error Resilient Video Coding by Dynamic Replacing of Reference Pictures” (S. Fukunaka et al.) (1996 IEEE) is known, but when a decoding error occurs, the previously decoded video signal is directly used again, the improving effect of picture quality deterioration was trifling.